Pressure swing adsorption process

ABSTRACT

A pressure swing adsorption process for the separation of nitrogen from a mixture of same with methane, utilizing two separate PSA stages, one containing a nitrogen selective adsorbent and the second containing a methane-selective adsorbent. In the process, the first PSA unit containing a nitrogen selective adsorbent forms a product methane stream and a waste stream rich in nitrogen which is passed to a second PSA unit containing a methane selective adsorbent which forms a product nitrogen stream and a waste stream rich in methane. The waste stream rich in methane can be treated to remove heavy hydrocarbons therefrom before the methane is recycled to feed.

FIELD OF THE INVENTION

This invention relates to the purification of natural gas, and, moreparticularly, to the removal of nitrogen from natural gas by use of amolecular sieve in a novel pressure swing adsorption (PSA) process.

BACKGROUND OF THE INVENTION

The removal of nitrogen from natural gas is of considerable importanceinasmuch as nitrogen is present to a significant extent. Nitrogencontamination lowers the heating value of the natural gas and increasesthe transportation cost based on unit heating value.

Applications aimed at removing nitrogen and other impurities fromnatural gas streams provide significant benefits to the U.S. economy. In1993, the Gas Research Insitute (GRI) estimated that 10-15% (˜22trillion cubic feet) of the natural gas reserves in the U.S. are definedas sub-quality due to contamination with nitrogen, carbon dioxide,sulfur. Most of these reserves, however, have discounted marketpotential, if they are marketable at all, due to the inability to costeffectively remove the nitrogen. Nitrogen and carbon dioxide are inertgases with no BTU value and must be removed to low levels (4% typically)before the gas can be sold.

Concurrently, the U.S. has proven reserves of natural gas totaling 167trillion cubic feet. Over the past five years, annual consumption hasexceeded the amount of new reserves that were found. This trend couldresult in higher cost natural gas and possible supply shortages in thefuture. As the U.S. reserves are produced and depleted, finding new,clean gas reserves involves more costly exploration efforts. Thisusually involves off shore exploration and/or deeper drilling onshore,both of which are expensive. Moreover, unlike crude oil, it is noteconomical to bring imports of natural gas into North America, thereforepricing of natural gas could be expected to rise forcing end users toseek alternative fuels, such as oil and coal, that are not as cleanburning as gas. While base consumption for natural gas in the U.S. isprojected to grow at 2-3% annually for the next ten years, one segmentmay grow much more rapidly. Natural gas usage in electric powergeneration is expected to grow rapidly because natural gas is efficientand cleaner burning allowing utilities to reduce emissions. Accordingly,there is a need to develop a cost-effective method of upgradingcurrently unmarketable sub-quality reserves in the U.S. therebyincreasing the proven reserve inventory.

Methods heretofore known for purification of natural gas, in particular,nitrogen removal, may be divided roughly into three classifications:

(a) Methods involving fractional distillation at low temperature and(usually) high pressure, i.e. cryogenics. Since nitrogen has a lowerboiling point than methane and the other hydrocarbons present in naturalgas, it may be removed as a gas on liquefying the remaining constituentswhich are then revaporized.

(b) By selective adsorption of the methane and higher hydrocarbons on anadsorbent such as activated charcoal. The adsorbed gases are thendesorbed to give a gas free of nitrogen.

(c) Miscellaneous processes involving selective diffusion through aseries of organic membranes, formation of lithium nitride by treatmentwith lithium amalgam, absorption of the nitrogen in liquid ammonia or inliquid sulphur dioxide.

The principal disadvantage of the fractional distillation and adsorptionprocesses is that they remove the major component, methane, from theminor component, nitrogen, instead of the reverse. In cryogenicprocessing, almost the entire volume of natural gas must berefrigerated, usually compressed, and then heated again. Accordingly,cryogenic processing is expensive to install and operate, limiting itsapplication to a small segment of reserves. Cryogenic technology isbelieved only capable of cost effectively purifying reserves, whichexceed 50,000,000 standard cubic feet of gas per day and as well havingnitrogen contamination levels of 15% or more. Gas reserves that do notfit these criteria are not currently being purified. The potential valueof this gas is totally lost as the wells are usually capped. Theprocesses suggested under paragraph (c) above are handicapped by anunsatisfactory degree of separation or by the use of very expensivematerials.

In smaller-scale natural gas operations as well as in other areas suchas synthesis gas and coke oven gas processing, adsorption processes canbe especially well suited. The adsorption capacities of adsorption unitscan, in many cases, be readily adapted to process gas mixtures ofvarying nitrogen content without equipment modifications, i.e. byadjusting adsorption cycle times. Moreover, adsorption units can beconveniently skid-mounted, thus providing easy mobility between gasprocessing locations. Further, adsorption processes are often desirablebecause more than one component can be removed from the gas. As notedabove, nitrogen-containing gases often contain other gases that containmolecules having smaller molecular dimensions than nitrogen, e.g., fornatural gas, carbon dioxide, oxygen and water, and for coke oven gas,carbon monoxide.

U.S. Pat. No. 2,843,219 discloses a process for removing nitrogen fromnatural gas utilizing zeolites broadly and contains specific examplesfor the use of zeolite 4A. The process disclosed in the patent suggestsuse of a first nitrogen selective adsorbent zeolite in combination witha second methane selective adsorbent. The molecular sieve adsorbent forremoving nitrogen is primarily regenerated during desorption by thermalswing. A moving bed adsorption/desorption process is necessary forproviding sufficient heat for the thermal swing desorption. The movingbed process specifically disclosed in this patent is not practical andit does not provide a cost efficient method for the separation ofnitrogen from natural gas in view of high equipment and maintenancecosts and degradation of the adsorbent by attrition due to contact withthe moving adsorbent particles.

Despite the advantageous aspects of adsorption processes, the adsorptiveseparation of nitrogen from methane has been found to be particularlydifficult. The primary problem is in finding an adsorbent that hassufficient selectivity for nitrogen over methane in order to provide acommercially viable process. In general, selectivity is related topolarizability, and methane is more polarizable than nitrogen.Therefore, the equilibrium adsorption selectivity of essentially allknown adsorbents such as large pore zeolites, carbon, silica gel,alumina, etc. all favor methane over nitrogen. However, since nitrogenis a smaller molecule than methane, it is possible to have a small porezeolite which adsorbs nitrogen faster than methane. Clinoptilolite isone of the zeolites which has been disclosed in literature as a rateselective adsorbent for the separation of nitrogen and methane.

U.S. Pat. No. 4,964,889 discloses the use of natural zeolites such asclinoptilolites having a magnesium cation content of at least 5equivalent percent of the ion-exchangeable cations in the clinoptilolitemolecular sieve for the removal of nitrogen from natural gas. The patentdiscloses that the separation can be performed by any known adsorptioncycle such as pressure swing, thermal swing, displacement purge ornonadsorbable purge, although pressure swing adsorption is preferred.However, this patent is silent as to specifics of the process, such assteps for treating the waste gas, nor is there disclosure of a highoverall system recovery.

In general, first applications of PSA processes were performed toachieve the objective of removing smaller quantities of adsorbablecomponents from essentially non-adsorbable gases. Examples of suchprocesses are the removal of water from air, also called heatlessdrying, or the removal of smaller quantities of impurities fromhydrogen. Later this technology was extended to bulk separations such asthe recovery of pure hydrogen from a stream containing 30 to 90 molepercent of hydrogen and other readily adsorbable components like carbonmonoxide or dioxide, or, for example, the recovery of oxygen from air byselectively adsorbing nitrogen onto molecular sieves.

The carrying out of the PSA processes in multi-bed systems isillustrated by the Wagner patent, U.S. Pat. No. 3,430,418, relating to asystem having at least four beds. As is generally known and described inthis patent, the PSA process is commonly performed in a cycle of aprocessing sequence that includes in each bed: (1) higher pressureadsorption with release of product effluent from the product end of thebed; (2) co-current depressurization to intermediate pressure withrelease of void space gas from the product end thereof; (3)countercurrent depressurization to a lower pressure; (4) purge; and (5)pressurization. The void space gas released during the co-currentdepressurization step is commonly employed for pressure equilizationpurposes and to provide purge gas to a bed at its lower desorptionpressure.

Similar systems are known which utilize three beds for separations. See,for example, U.S. Pat. No. 3,738,087 to McCombs. The faster the bedsperform steps 1 to 5 to complete a cycle, the smaller the beds can bewhen used to handle a given hourly feed gas flow. If two steps areperformed simultaneously, the number of beds can be reduced or the speedof cycling increased; thus, reduced costs are obtainable.

U.S. Pat. No. 4,589,888 to Hiscock, et. al. discloses that reduced cycletimes are achieved by an advantageous combination of specificsimultaneous processing steps. The gas released upon co-currentdepressurization from higher adsorption pressure is employedsimultaneously for pressure equalization and purge purposes. Co-currentdepressurization is also performed at an intermediate pressure level,while countercurrent depressurization is simultaneously performed at theopposite end of the bed being depressurized.

U.S. Pat. No. 4,512,780 to Fuderer discloses a pressure swing adsorptionprocess with intermediate product recovery. Three products are recoveredfrom a pressure swing adsorption process utilizing a displacement stepin conjunction with pressure equalization between beds of a multi-bedadsorption system. This process is not cost efficient for the recoveryof two products.

PSA processes were first used for gas separations in which only one ofthe key components was recovered at high purity. For example, from 100mols feed gas containing 80 mols hydrogen and 20 mols carbon monoxide,the process of Wagner, U.S. Pat. No. 3,430,418, or of Hiscock, et. al.,U.S. Pat. No. 4,589,888, could separate 60 mols of hydrogen at 99.999%purity, but no pure carbon monoxide could be recovered; 20 mols ofcarbon monoxide and 20 mols of hydrogen remained mixed at 50% purityeach. Neither of these processes can make a complete separation. Onlythe less adsorbable, light component is recovered at high purity.

For the recovery of a pure, stronger adsorbed, “heavy” component, anadditional step is necessary, namely, rinsing of the bed with a heavycomponent to displace the light component from the bed prior todepressurization. The rinsing step is described in several earlierpatents. The problems with these processes are the following: (a) if therinsing is complete and the light component is completely displaced fromthe bed, pure heavy component can be obtained, but the adsorption frontof the heavy component breaks through to the light component and thelatter cannot be recovered at high purity; (b) if the displacement ofthe light component is incomplete, the typical concentration profile ofthe heavy component in the bed is not optimum and such bed isdepressurized countercurrently to recover the heavy key component at thefeed end, the light component still present in the bed reaches the feedend very rapidly and the purity of the heavy component drops. Therefore,it is not practical with the prior art processes to obtain both keycomponents at high purity in a single PSA unit.

Such complete separations can be obtained, for example, by two separatepressure swing adsorption processing units wherein each unit includesseveral fixed beds. From a feed gas containing, for example, hydrogenand carbon monoxide (CO), the first unit recovers pure hydrogen and acarbon monoxide rich gas containing 70% carbon monoxide. This gasmixture is compressed and passed through a second PSA unit whichrecovers pure carbon monoxide and a hydrogen rich gas. The hydrogen richgas can be added as feed gas to the first PSA unit and then the cycle isrepeated. The combination of two independent PSA units can make anexcellent separation at very high flexibility. For example, from a gasmixture with two components this system can recover more than 99.8% ofthe adsorbable “light” component such as hydrogen at a purity of 99.999%and also recover essentially 100% of the more readily adsorbed componentsuch as carbon monoxide at a purity higher than 99.5%.

Although pressure swing separation adsorption (PSA) has been used toseparate a wide variety of gases, the simple fact remains that there isno commercially practiced PSA process for the separation of nitrogenfrom methane. This is due to many factors including the lack of anitrogen specific adsorbent and environmental regulations.

As previously pointed out, a significant percentage of U.S. natural gasreserves contain more than 4% nitrogen. The bulk of these reservescannot be exploited because no economical technology exists for removingnitrogen especially at low flow rates, i.e., less than 25 MMSCFD processfeed gas. Cryogenic distillation is the only process being used to dateon any scale to remove nitrogen from methane in natural gas. Cryogenicplants are not used more widely because they are expensive andcomplicated and exhibit poor scale down economics.

It is the primary objective of this invention to provide a commerciallyviable PSA process for removing nitrogen from natural gas.

A further object of the invention is to provide a PSA process forremoving nitrogen from natural gas which can provide a highlyconcentrated methane product at high process efficiencies.

Another object of this invention is to separate nitrogen from naturalgas by a novel PSA process which yields a methane product, and a highpurity nitrogen stream.

Still another object of the invention is to provide and maintain peakefficiency of the nitrogen-selective adsorbent during PSA separation ofnitrogen from natural gas.

SUMMARY OF THE INVENTION

This invention provides a PSA system to achieve separation of nitrogenfrom natural gas with high system recovery of separate methane andnitrogen product streams from the feed gas. This is accomplished byplacing a methane selective adsorbent in a PSA system on the wastestream of a nitrogen-selective PSA system in order to boost the methanerecovery of the system and further generate a high purity nitrogen gasstream that could be utilized for other related unit operations orvented to the atmosphere.

In another aspect of the invention, the methane-enriched waste streamfrom the methane selective PSA is cooled to remove C₃+ hydrocarbonsbefore being recycled to feed so as to eliminate the build up of heavyhydrocarbons in the PSA units.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 represents a PSA process with recycle of the waste gas where onlya nitrogen selective crystalline zeolite is used.

FIG. 2 represents a PSA process utilizing both a nitrogen selectivecrystalline zeolite and a methane selective adsorbent wherein themethane is recycled back to the feed gas.

FIG. 3 is substantially similar to FIG. 2 except a fuel generation stepis introduced into the PSA cycle.

FIG. 4 is substantially similar to FIG. 3 except the stream designatedby W2 is routed to Fuel 2.

FIG. 5 is a graph of product purity v. time for a fixed product rateusing a nitrogen-selective adsorbent.

FIG. 6 is similar to the process scheme of FIG. 2, except the methanerecycle is first treated to remove heavy hydrocarbons.

DETAILED DESCRIPTION OF THE INVENTION

As is known in the prior art, natural gas streams frequently containcomponents smaller than nitrogen, such as water vapor, carbon dioxideand hydrogen sulfide. The gas stream to be treated in accordance withthe novel process of this invention preferably would have thesecontaminants removed prior to treatment of the feed gas stream inaccordance with the novel process of this invention.

The amount of nitrogen present in said feed gas stream is not criticalin carrying out the novel process of this invention and can be as low as1 mol percent or as high as about 65 mol percent. Typically, thenitrogen content is in the range of 5 to 30 mol percent.

As has been heretofore stated, FIG. 1 is a schematic illustration ofusing only a nitrogen selective crystalline zeolite adsorbent. As can beseen from FIG. 1, a mixture F1 of a feed gas F and recycle W1 isintroduced into an adsorption column 1 containing a nitrogen selectivezeolite molecular sieve. From the top of the column 1 a methane productP is recovered and from the bottom of the column a waste W is recoveredand may be compressed and split into two fractions, a recycle fractionlabeled as W1 and a waste fraction identified as W2.

FIG. 2 represents an illustration of a novel process of this inventionwhere a first PSA system depicted as column 2, is used involving anitrogen selective molecular sieve and wherein the waste Wl is treatedin a second PSA process, depicted in column 3, involving the use of amethane selective adsorbent. The identification in FIG. 2 is the same asin FIG. 1 with the sole exception that the waste from the PSA column 2is treated with a methane selective adsorbent and product fractionsidentified as NP (nitrogen product) and P (methane product) arerecovered, and a recycle fraction identified as W2 is recovered andrecycled to the PSA column 2.

FIG. 3 represents a modification of FIG. 2 in that a fuel fraction isobtained from the top of the PSA column 2 and used as fuel.

FIG. 4 represents a modification of FIG. 3 in that the stream designatedas W2 is routed to Fuel 2, i.e., two fuel streams are produced.

The following Table 1 compares the Material Balances of the variousprocesses set forth in FIGS. 1, 2, 3, and 4.

In the table, the nitrogen selective PSA unit was run under rateadsorption conditions and the methane selective PSA was run underequilibrium adsorption conditions as shown in the Figures. The preferrednitrogen selective adsorbent used is believed to be size selective fornitrogen and thus can be used under equilibrium conditions with similarrecoveries as shown in Table 1.

TABLE 1 Material Balances Tables FIG. 1 Rate PSA System With Recycle FF1 P W W1 W2 Flow (MMSCFD) 15 33.75 11.25 22.5 18.75 3.75 mol % C175.00% 40% 96.00% 12.00% 12.00% 12.00% Single Pass 80% Recovery OverallPlant 96.00% Recovery FIG. 2 Rate and Equ PSA F F1 P W1 W2 NP Flow(MMSCFD) 15 18.46 11.58 6.89 3.46 3.42 mol % C1 75.00% 75.24% 96.00%40.34% 76.27% 4.00% Single Pass 80% Recovery Overall Plant 98.78%Recovery FIG. 3 Rate and Eq PSA with fuel F F1 P W1 W2 NP Fuel Flow(MMSCFD) 15 16.92 11.58 5.34 1.92 3.42 1.16 mol % C1 75.00% 72.98%96.00% 23.11% 57.19% 4.00% 96.00% Single Pass 80% Recovery Fuel 10%Overall Plant 98.78% Recovery including fuel FIG. 4 Rate and Eq PSA norecycle two fuel streams F P W Fuel 1 NP Fuel 2 Flow (MMSCFD) 15 9.385.04 0.59 2.79 2.25 mol % C1 75.00% 96.00% 33.49% 96.00% 4.00 70.16%Single Pass 80% Recovery Fuel 19%

As can be seen from the above Table 1, in FIG. 1 a feed F containing amixture of 75 mol percent methane and 25 mol percent nitrogen isintroduced into a rate PSA column together with a recycle streamidentified as W1, to form a feed mixture F1 which contains 40% ofmethane. It can be seen from the Material Balance Table in connectionwith FIG. 1 that the single pass recovery is 80%. Single pass recoveryis defined as methane mol fraction in the gas phase, multiplied by themols of product per hour divided by the feed methane mol fraction gasphase times the mols of product per hour. Thus, in connection with FIG.1 the product contains 96% of methane times a flow rate of 11.25 MMSCFDdivided by 40% methane times 33.75 MMSCFD in order to obtain a singlepass recovery of 80%. Overall plant recovery is obtained from P flowrate times P methane composition divided by F times F's methanecomposition and is 96%.

Although the amount of methane recovery in terms of percentage is high,nevertheless the process set forth in FIG. 1 generally requires theprocessing of an extremely large recycle stream identified as W1. Infact the recycle stream which is being processed is greater than thefeed stream. Quite obviously, the economics of this process needimprovement.

Reference to the above material balance in Table 1 in connection withFIG. 2, which represents the novel process of this invention, will showthat the single pass recovery is the same; however, the overall plantrecovery increases from 96% to 98.78%. Far more importantly, the recyclestream W2 is significantly reduced compared to the recycle stream inFIG. 1, i.e., 3.46 as opposed to 18.75 MMSCFD. The productivity of theunit is inherently increased because less of a recycle stream needs tobe mixed with the feed stream.

FIG. 3 represents an embodiment in the novel process of this inventionwherein upon need of a fuel requirement in the system, a vent step canbe placed in the rate PSA cycle. As can be seen from the MaterialBalance Table, this further reduces the recycle flow of W2 down to 1.92MMSCFD.

The temperature of the rate PSA (first stage) is preferably maintainedin the range of from about −50° to +100° C., and more preferably from 0°to 70° C. The pressure during the adsorption is from about 20 psia to2,000 psia, and preferably about 100 to 1,500 psia and more preferablyfrom 500 to 1,000 psia. The pressure during desorption being lower thanduring adsorption and effective to cause the desorption of nitrogen, ispreferably from about 1 to 150 psia, more preferably from about 5 to 50psia and most preferably from about 5 to 25 psia. The cyclic process cancomprise additional adsorption and regeneration steps as well asintermediate depressurization and purging steps, as is well-known in theart.

As can be seen from Table 1, the product from this first stage PSAcontains 96 mol percent of methane and only 4 mol percent of nitrogen.Quite obviously, the product is a quality fuel. However, the purge orwaste from this first stage PSA contains 12 mol percent of methane and88 mol percent of nitrogen. Quite obviously, it cannot be used as a fueldue to its high nitrogen content nor can it be discharged to theatmosphere. Such discharge is not in the best interest of theenvironment and it also involves a waste of methane.

Thus, in accordance with this invention, as shown in FIG. 2 and FIG. 3,the waste or purge stream from the first stage of a PSA is fed to asecond stage PSA containing an adsorbent which is selective for methane.Benefits of such processing are shown in Table 1.

The second stage PSA operates at a temperature of −30° to 140° F.,preferably 70° to 120° F. and at a pressure of from 1 to 2,000 psia,preferably 5 to 60 psia utilizing an adsorbent selective for methane aswill be later defined.

The waste or purge stream can be recycled back to the feed stream asshown in FIGS. 2 and 3 or used as fuel as shown in FIG. 4.

FIG. 6 represents an improvement over the process set forth in FIGS. 2and 3 wherein the methane-enriched waste stream from the second PSA unitis recycled to feed. What has been found is that by the continuousrecycle of the methane-enriched waste stream W2 to the feed stream, boththe first and second PSA units accumulate the heavier hydrocarboncomponents including C₃+ materials. These materials can continuallybuild up in the recycle stream. The process scheme shown in FIG. 6alleviates this problem by removing the heavier hydrocarbons from themethane enriched stream W2 prior to recycle to feed. Thus, as shown inFIG. 6 the stream W2 subsequent to recycle compression is refrigeratedto a temperature of from about −30° to about 30° C. and the cooledmethane enriched stream W2 is then forwarded to a knock out separator KOto separate the heavy hydrocarbons in liquid form from the lightercomponents including methane which can be recycled as a gas to the feedF. Typically, the C₃+ hydrocarbons will comprise from about 1 to about25 mol % of stream W2. The reduction of these heavy hydrocarbons fromthe recycle stream W2 reduces the level that these hydrocarbonsaccumulate in the PSA processes.

The Nitrogen Selective Crystalline Zeolite

The nitrogen selective crystalline zeolites utilized in the first stagePSA are either CTS-1 zeolites described and claimed in the U.S. Pat. No.6,068,682, issued May 30, 2000 and assigned to Engelhard Corporation, orbarium exchanged ETS-4 described and claimed in the U.S. Pat. No.5,989,316, issued Nov. 23, 1999 and again assigned to present assigneeEngelhard Corporation.

The CTS-1 zeolites are characterized as having a pore size ofapproximately 3-4 Angstrom units and a composition in terms of molratios of oxide as follows:

1.0±0.25 M_(2/n)O:TiO₂:ySiO₂:zH₂O

wherein M is at least one cation having a valence n, y is from 1.0 to100 and z is from 0 to 100, said zeolite being characterized by thefollowing X-ray diffraction pattern.

D-spacings (Angstroms) I/I₀ 11.3 ± 0.25 Very Strong 6.6 ± 0.2Medium-Strong  4.3 ± 0.15 Medium-Strong 3.3 ± 0.07 Medium-Strong 2.85 ±0.07 Medium-Strong

wherein very strong equals 100, medium-strong equals 15-80.

The CTS-1 materials are titanium silicates which are different thanconventional aluminosilicate zeolites. The titanium silicates usefulherein are crystalline materials formed of octahedrally coordinatedtitania chains which are linked by tetrahedral silica webs. The CTS-1adsorbents are formed by heat treating ETS-4 which is described in U.S.Pat. No. 4,938,939, issued Jul. 3, 1990 and assigned to EngelhardCorporation. U.S. Pat. Nos. 4,938,939; 5,989,316; and 6,068,682 areherein incorporated by reference.

Barium ETS-4 is ETS-4 which has been exchanged with barium such thatbarium represents at least 30% of the exchangeable cations of ETS-4.

The Methane Selective Crystalline Zeolite

The methane selective adsorbent used in the second stage PSA is either acrystalline aluminosilicate zeolite such as 13X or a high aluminum Xhaving a silicon-to-aluminum ratio of about 1 or an amorphous adsorbentsuch as silica gel or carbon.

It is preferred to employ the high aluminum X zeolite in the sodium formalthough other exchanged forms can be used.

A particularly preferred high aluminum X is zeolite XE whosepreparations will be shown in the examples.

The most preferred methane adsorbent is carbon.

As is known in the PSA art, the zeolites are composited or grown in-situwith materials such as clays, silica and/or metal oxides. The latter maybe either naturally occurring or in the form of gelatinous precipitatesor gels including mixtures of silica and metal oxides. Normallycrystalline materials have been incorporated into naturally occurringclays, e.g., bentonite and kaolin, to improve the crush strength of thesorbent under commercial operating conditions. These materials, i.e.,clays, oxides, etc., function as binders for the sorbent. It isdesirable to provide a sorbent having good physical properties becausein a commercial separation process, the zeolite is often subjected torough handling which tends to break the sorbent down into powder-likematerials which cause many problems in processing. These clay bindershave been employed for the purpose of improving the strength of thesorbent.

Naturally occurring clays that can be composited with the crystallinezeolites include the smectite and kaolin families, which familiesinclude the montmorillonites such as sub-bentonites and the kaolinsknown commonly as Dixie, McNamee, Georgia and Florida or others in whichthe main constitutent is halloysite, kaolinite, dickite, nacrite oranauxite. Such clays can be used in the raw state as originally mined orinitially subjected to calcinations, acid treatment or chemicalmodification.

In addition to the foregoing materials, the crystalline zeolites may becomposited with matrix materials such as silica-alumina,silica-magnesia, silica-zirconia, silica-thoria, silica-berylia,silica-titania as well as ternary compositions such assilica-alumina-thoria, silica-alumina-zirconia, silica-alumina-magnesiaand silica-magnesia-zirconia. The matrix can be in the form of a cogel.The relative proportions of finally divided crystalline metalorganosilicate and inorganic oxide gel matrix can vary widely with thecrystalline organosilicate content ranging from about 5 to about 90percent by weight and more usually in the range of 90 percent by weightof the composite.

Prior to being used, the adsorbents are thermally treated as iswell-known in the art.

If carbon or silica gel is used as the methane selective adsorbent, theyneed not be composited with the aforementioned materials.

The following examples will illustrate the novel processes of thisinvention.

EXAMPLE 1

In this Example, reference to FIG. 2 and Table 2 below is made. A feedgas of approximately 1 MMSCFD is introduced into a bed containingapproximately 60 ft3 of adsorbent. The adsorbent consists of beads ofCTS-1 or BA-ETS-4 zeolite with a mesh range of 4 to 40. The feed gas F1is fed for a period of approximately 80 seconds at a pressure ofapproximately 400 psia. The stream leaving the top of the PSA column 2at this point is designated P in Table 2 below. At the end of 80seconds, the feed supply is closed off. The bed is then depressurizedco-currently to another bed or a tank in a step referred in the art asequalization. The pressure at the end of the first equalization isapproximately 240 psia. The bed is further depressurized co-currently toa pressure of 120 psia to a second bed or tank in an additionalequalization. The two equalization steps are allowed approximately 20seconds each to complete. After the bed has completed two equalizationsit is further co-currently depressurized to 75 psia to provide purge gasto another bed. The purge gas can be stored temporarily in a tank orallowed to go directly into another bed. The providing purge gas step isgiven approximately 20 seconds to complete. The bed is thencounter-currently depressurized to a pressure of approximately 5 psiafor a period of approximately 10 seconds (blow down gas). Next the bedis purged counter currently with a gas from either a tank previouslyfilled from the provide purge step, or another bed undergoing aco-current depressurization. The purge step takes approximately 100seconds to complete. The gas leaving the bed during the purge step gasand previous blow down gas steps are combined to create the streamdesignated W. The composition and flow of the stream designated W can befound in Table 2. Then the bed is pressurized with gas from the previousequalization steps and brought back up to a pressure of approximately240 psia pressure. Subsequently the bed is pressurized counter-currentlywith product gas, or alternatively pressurized with feed gasco-currently.

The rate process waste stream W is compressed to a pressure ofapproximately 50 psia to form stream W1. This stream is then fed to asecond PSA holding 40 ft3 of adsorbent where it is fed for a period ofapproximately 160 seconds. The gas leaving the second PSA bed duringthis step is designated “NP” and has a flow and composition asdesignated in Table 2. Then equalization to a second bed is completed inthe co-current direction at which point the pressure is approximately 28psia. The bed is further co-currently depressurized to approximately 15psia to provide a purge gas for subsequent use. Next the bed isdepressurized counter-currently (blown down) to a pressure of 5 psia.The bed is then purged with gas provided from a second bed or tank at 5psia. The gas leaving the bed during the blow down and purge steps arecombined to give the “W2” stream. Finally, the bed is brought back up topressure by counter-currently pressurizing to the adsorption pressureusing equalization gas and product gas. The equalization and providepurge steps are given approximately 20 seconds each to complete. Thepurge step is given approximately 80 seconds to complete.

Material balances for the two units are given in Table 2 below. Thoseskilled in the art will recognize that the process can be defined by theboundary flows about the unit, the number of equalizations, and providepurge ending pressure. This information gives sufficient information forthose skilled in the art to practice said invention. As can be seen inTable 2, the rate PSA recovers 80% of the methane in the feed gas in asingle pass configuration. By further processing the rate PSA wastestream it is possible to recover 93.7% of the methane leaving the rateprocess at a composition close to the feed composition. This streamidentified as “W2” can then be recycled back to the feed and of theprocess to bring the overall plant recovery to 98.74%, a significantimprovement over the 80% single pass recovery.

TABLE 2 F P W NP W2 Flow (MMSCFD) 1 0.625 0.375 0.189474 0.185526Pressure (psia) 400 400 5 50 5 Temperature (F.) 80 90 60 85 60 Comp (mol75%   96% 40.00%   5% 75.74% %) C1 Comp (mol 25%    4% 60.00%  95%24.26% %) N2 Rec C1   80%  20.0% 6.3%  93.7% Overall C1 Rec 98.74%

TABLE 3 F P Fuel W NP W2 Flow 1 0.625 0.46875 0.328125 0.179688 0.148438(MMS- CFD) Pressure 400 400 50 5 50 5 (psia) Tempera- 80 90 80 60 85 60ture (F.) Comp 75%   96% 80% 34%   5% 69.74% (mol %) C1 Comp 25%    4%20% 66%  96% 30.26% (mol %) N2 Rec C1   80%  5% 1.2%  13.8% Overall98.80% C1 Rec

EXAMPLE 2

In this Example, reference is made to FIG. 3 and Table 3. A feed gas ofapproximately 1 MMSCFD is introduced into a bed containing approximately60 ft3 of adsorbent. The adsorbent consists of beads of CTS-1 orBA-ETS-4 zeolite with a mesh range of 4 to 40. The feed gas is fed for aperiod of approximately 80 seconds at a pressure of approximately 400psia. The stream leaving the top of the PSA column 2 at this point isdesignated P in Table 3. At the end of 80 seconds, the feed supply isclosed off. The bed is then depressurized co-currently to another bed ora tank in a step referred in the art as equalization. The pressure atthe end of the first equalization is approximately 240 psia. The bed isfurther depressurized co-currently to a pressure of 120 psia to a secondbed or tank in an additional equalization. The two equalization stepsare allowed approximately 20 seconds each to complete. After the bed hascompleted two equalizations it is further co-currently depressurized to90 psia to generate a fuel quality stream. The fuel stream is valuableas an energy source for other processing equipment, includingcompression dehydration and CO₂ removal processes. The bed is furtherco-currently depressurized to 70 psia to provide a purge gas to anotherbed. The purge gas can be stored temporarily in a tank or allowed to godirectly into another bed. The providing purge gas step is givenapproximately 20 seconds to complete. The bed is then counter-currentlydepressurized to a pressure of approximately 5 psia for a period ofapproximately 10 seconds (blow down gas). Next the bed is purged countercurrently with a gas from either a tank previously filled from theprovide purge step, or another bed undergoing a co-currentdepressurization. The purge step takes approximately 100 seconds tocomplete. The gas leaving the bed during the purge step gas and previousblow down gas steps are combined to create the stream designated W. Thecomposition and flow of the stream designated W can be found in Table 3.Then the bed is pressurized with gas from the previous equalizationsteps and brought back up to a pressure of approximately 240 psiapressure. Subsequently, the bed is pressurized counter-currently withproduct gas, or alternatively pressurized with feed gas co-currently.

The rate process waste stream W is compressed to a pressure ofapproximately 50 psia to form stream W1. Stream Wl is then fed to asecond PSA holding 40 ft3 of adsorbent where it is fed for a period ofapproximately 160 seconds. The gas leaving the bed during this step isdesignated “NP” and has a flow and composition as designated in Table 3.Then equalization to a second bed is completed in the co-currentdirection at which point the pressure is approximately 28 psia. The bedis further co-currently depressurized to approximately 15 psia toprovide a purge gas for subsequent use. Next the bed is depressurizedcounter-currently (blown down) to a pressure of 5 psia. The bed is thenpurged with gas provided from a second bed or tank at 5 psia. The gasleaving the bed during the blow down and purge steps are combined togive the “W2” stream. Finally, the bed is brought back up to pressure bycounter-currently pressurizing to the adsorption pressure usingequalization gas and product gas. The equalization and provide purgesteps are given approximately 20 seconds each to complete. The purgestep is given approximately 80 seconds to complete. Product recoveriesare set forth in Table 3.

EXAMPLE 3

The performance of the rate PSA as given in Example 1 represents averageflows and compositions over a 24 hour time frame. After an initial 1hour startup period the rate PSA starts producing a higher puritymethane product stream P than the average purity as described in Table2, subsequently after 8 hours the purity of the product stream dropsbelow 96% purity. This phenomenon of a gradual degradation inperformance is illustrated in FIG. 5. FIG. 5 shows the methane productpurity v. time for a fixed feed flow rate. As can be seen in FIG. 5 theproduct purity v. time is dropping.

Periodically heating the bed increases the nitrogen working capacity(amount of nitrogen desorbed each cycle) of the rate PSA; it is believedthat this is accomplished by lowering the methane loading on theadsorbent. The loss in nitrogen working capacity is illustrated by thelowering of product purity at a fixed feed rate. This performancedecline v. time can be mitigated by periodically heating a bed(s) in therate PSA with gas flow from stream NP of Table 2. For example, thisstream can be used to heat the rate PSA to 200° F. for 1.5 hours andthen cool the rate PSA for 1.5 hours to 70° F. After the cooling periodhas been completed, the bed in the rate PSA is again fed feed gas. Otherstreams including the methane product stream P and waste stream W1 canbe used to heat the nitrogen-selective adsorbent. If stream P is used, atemperature of about 300° F. ensures that the methane is desorbed fromthe sorbent. The preferred method of cooling is to use nitrogen productstream NP. The periodic heating of the nitrogen selective adsorbent hasbeen found to be particularly useful for the CTS-1 adsorbent.

EXAMPLE 4 Preparation of Potassium Exchanged XE Zeolite Beads(“Equilibrium”) Adsorbent

I. XE Zeolite Synthesis

The following reagents were added to a 1,000 gallon SS reactor: 904 kgof DI water, 435 kg of a sodium hydroxide solution (38.6% Na₂O), 578 kgof a potassium hydroxide solution (37.3% K₂O), 1,250 kg of N-Clearsodium silicate (28.7% SiO2/8.93% Na₂O) and 1,300 kg of Nalco 2372sodium aluminate (19.9% Al₂O₃/18.1% Na₂O) while stirring at ˜75 rpm.This stirred slurry was then heated to 75° C. and reacted for 20 hours.The resulting product slurry was filtered on a plate and frame filterpress then washed with 1,000 gallons of DI water at 75° C. Thisinitially washed cake was reslurried in 1,000 gallons of DI water andthen heated to 75° C. for 60 minutes. The reslurry was filtered on theplate and frame filter press and then finally washed with 2,000 gallonsof DI water at 75° C. This washed XE zeolite cake was then potassiumexchanged as follows:

II. Preparation of Potassium Exchanged XE Zeolite

a. 1^(st) Exchange: A 25% potassium chloride solution was prepared asfollows: 980 kg of KC1 was dissolved in 3,000 kg of DI water in astirred (at ˜75 rpm) 2,000 gallon SS reactor. To this solution was addedthe washed XE zeolite cake from Step #I. While stirring at ˜75 rpm, thisexchange slurry was reacted at 75° C. for 90 minutes. The resultingslurry was filtered on a plate and frame filter press and then washedwith 1,500 gallons of DI water at 75° C. This washed cake was furtherpotassium exchanged as follows:

b. 2^(nd) Exchange: A second 25% potassium chloride solution wasprepared as follows: 980 kg of KC1 was dissolved in 3,000 kg of DI waterin the stirred (at ˜75 rpm) 2,000 gallon SS reactor. To this solutionwas added the washed cake from step #IIa. While stirring at ˜75 rpm,this exchange slurry was reacted at 75° C. for 90 minutes. The resultingslurry was filtered on a plate and frame filter press and then washedwith 1,500 gallons of DI water at 75° C. This second washed cake wasfinally potassium exchanged as follows:

c. Final Exchange: A 25% potassium chloride solution was prepared asfollows: 980 kg of KC1 was dissolved in 3,000 kg of DI water in thestirred (at ˜75 rpm) 2,000 gallon SS reactor. To this solution was addedthe washed cake from step #IIb. The pH of this slurry was then adjustedto 11.0 by the addition of an approximate amount of 45% KOH solution.While stirring at ˜75 rpm, this final exchange slurry was reacted at 75°C. for 90 minutes. The resulting slurry was filtered on the plate andframe filter press and then washed with 3,000 gallons of DI water at 75°C. This washed potassium exchanged XE zeolite cake was reslurried in anapproximate amount of DI water and then spray dried into a powder.

III. Preparation of Dense 10% Bentonite Bound Beads (⅛″) of PotassiumExchanged XE Zeolite

1,360 lbs. (solids basis) of the spray dried potassium exchanged XEzeolite powder from step #IIc was dry blended with 240 lbs. (solidsbasis) Volclay SPV 200 bentonite powder in an appropriate sized pugmill. After the powders were thoroughly mixed, enough DI water was addedto the pug mill to produce a “good” extrusion dough. The dough mixturewas extruded into ⅛″ pellets using a twin barrel extruder then dried at110° C. overnight. These dried pellets were then reworked in the pugmill by the addition of sufficient DI water to again produce a “good”extrusion dough. This reworked dough mixture was extruded into ⅛″pellets using the twin barrel extruder, then beaded into ⅛″ spheresusing an appropriately sized Marumerizer. The “green” beads were traydried at 100° C., then activated at 250° C. in a rotary calciner. Theresulting 40 ft³ of “equilibrium” adsorbent beads were sealed in 55gallon drums.

EXAMPLE 5 Preparation of CTS-1 Beads (“Rate Adsorbent”)

I. ETS-4 Molecular Sieve Synthesis

a. Gel Preparation: A caustic solution was prepared by blending together2,600 lbs. of DI water, 6,806 lbs. of N-Clear sodium silicate (28.7%SiO₂/8.93% Na₂O) and 6.766 lbs. of sodium hydroxide solution (38.6%Na₂O) in a stirred 4,000 gal. tank. An acidic solution of equal volumewas prepared by blending together 3,819 lbs. of DI water, 8,395 lbs. oftitanium sulfate solution (10.3% TiO₂/36.7% H₂SO₄) and 631 lbs. ofsulfuric acid (96.7% H₂SO₄) in a second stirred 4,000 gal. tank. Thesetwo solutions were then simultaneously added at ˜10 gpm each into a 100gal. stirred (1,300 rpm) strike tank. The resulting gel was pumped intoa 5,000 gal. holding tank at a rate which maintained ˜70 gal. of gel inthe strike tank.

b. Gel Crystallization to ETS-4: 900 lbs. of the above gel were added toa stirred (˜75 rpm) 100 gal. titanium clad stainless steel (SS)autoclave then reacted at 215° C. for 24 hours. 452 lbs. of theresulting product slurry were filtered on a 1.2 ft³ plate and framefilter press, then washed with 75 gal. of DI water at 170° F. Thisinitially washed cake was then reslurried (at ˜50 rpm) in 75 gal. of DIwater in a 100 gal. SS reactor and heated to 170° F. for 15 min. Thereslurry was filtered on the plate and frame filter press and thenfinally washed with 150 gal. of DI water at 170° F. This washed ETS-4cake (18.5% Na₂O/54.2% SiO₂/27.8% TiO₂) was then strontium exchanged asfollows:

II. Preparation of Strontium Exchanged ETS-4 Molecular Sieve CTS-1

7.84 kg of SrCl₂. 6H₂O was dissolved in 34 gal. of DI water in the 100gal. SS reactor. To this solution was added 39.7 kg of the above ETS-4filter cake which equals 15.7 kg ETS-4 on a dry basis (as determined byan Ohaus moisture analyzer (Model #6010PC). While stirring at ˜50 rpm,this exchange slurry was reacted at 170° F. for 90 min. The resultingproduct slurry was filtered on the 1.2 ft³ plate and frame filter pressand then washed with 150 gal. of DI water at 170° F. This washed (Sr/Na)ETS-4 cake (4.37% Na₂O/20.3% SrO/50.7% SiO₂/23.3% TiO₂) was then driedat 110° C.

III. Preparation of Dense 10% Bentonite Bound Beads (˜12/+40 Mesh) ofCTS-1

1,715 g of the above (Sr/Na) ETS-4 dried cake were added to the bowl ofa 12″ diameter Eirich blender (Model #R02). This equals 1,650 g (drybasis) as determined by an Ohaus moisture analyzer (Model #6010PC).Next, 196.1 g of bentonite clay powder (Volclay SPV 200) were added tothe Eirich bowl. This equals 156.9 g (dry basis) as determined by theOhaus moisture analyzer. These two dry powders were then mixed for ˜10minutes on the low rotation setting #I and low agitation setting #I.

DI water was then added to the blended powder while still mixing on thelow rotation and agitation settings. The water was added a portion at atime, with reduced amounts being added as the mixture got “wetter”. Thetotal amount of water added was 1,550 g. The bowl was then rotated onthe high setting #II until mostly “oversized”, i.e., +12 mesh sized,product was obtained. Occasionally, the agitator was turned on (at thelow setting #I) to reduce larger chunks. The resulting “oversized” beadswere dried at 110° C. overnight, then reworked as follows:

DI water was added to the dried beads while mixing on the low rotationand agitation settings. Again, the water was added a portion at a time,with reduced amounts being added as the mixture got “wetter”. 1,260 g ofwater was added during this step. The bowl was then rotated on the highsetting #II until mostly −12/+40 mesh product was obtained.Occasionally, the agitator was turned on (at the low setting #I) toreduce the larger beads. “Oversized” beads were separated by screeningwith a 12 mesh screen, then reworked. When the entire product passedthrough the 12 mesh screen, it was dried overnight at 100° C. The driedbeads were then classified using 12 and 40 mesh screens. The totalweight of dried −12/+40 mesh beads obtained was 1,196 g.

Glossary of Terms

Rate Selectivity is defined as to assume that equal concentrations ofcomponent A and B exist above a clean adsorbent at time zero. Ifcomponent A adsorbs at a faster ratio than component B then theadsorbent is rate selective for component A.

Equilibrium Selectivity is defined as to assume that equalconcentrations of component A and B exist above an adsorbent. Further,the adsorbed phase concentration is not changing in time, and the gasphase concentration is not changing as a function of time. If componentA adsorbs to a higher concentration in the adsorbed phase than componentB, then the adsorbent is equilibrium selective for component A.

Rate PSA is defined as a PSA that exploits a rate selective adsorbent.

Equilibrium PSA is defined as a PSA that exploits an equilibriumadsorbent.

Once given the above disclosure, many other features, modifications, andimprovements will become apparent to the skilled artisan. Such otherfeatures, modifications, and improvements are, therefore, considered tobe a part of this invention, the scope of which is to be determined bythe following claims.

We claim:
 1. A pressure swing adsorption (PSA) process for theseparation of nitrogen from a mixture of the same with methane whichcomprises: (a) passing a feed stream comprising said mixture to a firstPSA unit containing a nitrogen selective adsorbent so as topreferentially adsorb nitrogen and produce a product stream containingat least 90 mol % methane and a purge stream; (b) recovering saidproduct stream; (c) passing said purge stream to a second PSA unitcontaining a methane selective adsorbent so as to produce a productstream rich in nitrogen and a purge stream rich in methane; (d)recovering said nitrogen-rich product stream; and (e) treating saidpurge stream rich in methane so as to remove heavy hydrocarbons fromsaid purge stream rich in methane.
 2. The process of claim 1, whereinsaid purge stream rich in methane is treated by cooling so as to liquefythe heavy hydrocarbons and, subsequently separating said heavyhydrocarbons from methane.
 3. The process of claim 1, wherein said purgestream rich in methane after treatment in step (e) is recycled to saidfeed stream.
 4. The process of claim 1, wherein said heavy hydrocarbonsremoved from said purge stream rich in methane include C₃+ hydrocarbons.5. The process of claim 1, wherein said heavy hydrocarbons comprise C₃+hydrocarbons, said purge stream rich in methane containing at leastabout 1 mol % of said C₃+ hydrocarbons.
 6. The process of claim 5,wherein said purge stream rich in methane after treatment in step (e) isrecycled to said feed stream.
 7. The process of claim 1, wherein thenitrogen selective adsorbent of step (a) is a CTS-1 zeolitecharacterized as having a pore size of approximately 3-4 Angstrom unitsand a composition in terms of mol ratios of oxide as follows: 1.0±0.25M_(2/n)O:TiO₂:ySiO₂:zH₂O wherein M is at least one cation having avalence n, y is from 1 to 100, and z is from 0 to 100, said zeolitebeing characterized by the following X-ray diffraction pattern.D-spacings (Angstroms) I/I₀ 11.4 ± 0.25 Very Strong 6.6 ± 0.2Medium-Strong  4.3 ± 0.15 Medium-Strong 3.3 ± .07 Medium-Strong 2.85 ±.07  Medium-Strong


8. The process of claim 7, wherein the methane selective adsorbent ofstep (c) is selected from the group consisting of a high aluminum Xhaving a silicon-to-aluminum ratio of about 1, zeolite 13X, carbon orsilica gel.
 9. The process of claim 1, wherein the nitrogen selectiveadsorbent of step (a) is barium exchanged ETS-4 wherein bariumrepresents at least 30% of the exchangeable cations.
 10. The process ofclaim 1, wherein the methane selective adsorbent of step (c) is selectedfrom the group consisting of a high aluminum X having asilicon-to-aluminum ratio of about 1, zeolite 13X, carbon or silica gel.11. The process of claim 1, wherein a co-current depressurization stepis introduced into said first PSA unit to generate a fuel stream. 12.The process of claim 1, wherein said first PSA unit operates under theconditions of rate selectivity.
 13. The process of claim 12, whereinsaid second PSA unit operates under the conditions of equilibriumselectivity.
 14. The process of claim 1, wherein subsequent to step (c)periodically heating said nitrogen selective adsorbent to drive offaccumulated methane therefrom.
 15. The process of claim 14, wherein thenitrogen selective adsorbent of step (a) is a CTS-1 zeolitecharacterized as having a pore size of approximately 3-4 Angstrom unitsand a composition in terms of mol ratios of oxide as follows: 1.0±0.25M_(2/n)O:TiO₂:ySiO₂:zH₂O wherein M is at least one cation having avalence n, y is from 1 to 100, and z is from 0 to 100, said zeolitebeing characterized by the following X-ray diffraction patternD-spacings (Angstroms) I/I₀ 11.4 ± 0.25 Very Strong 6.6 ± 0.2Medium-Strong  4.3 ± 0.15 Medium-Strong 3.3 ± .07 Medium-Strong 2.85 ±.07  Medium-Strong.


16. The process of claim 14, wherein the nitrogen selective adsorbent ofstep (a) is barium exchanged ETS-4 wherein barium represents at least30% of the exchangeable cations.
 17. The process of claim 14, whereinthe nitrogen-selective adsorbent is periodically heated with thenitrogen rich product stream of said second PSA unit.
 18. The process ofclaim 14, wherein said nitrogen-selective adsorbent is periodicallyheated with said purge stream from said first PSA unit.
 19. The processof claim 14, wherein said nitrogen selective adsorbent is periodicallyheated with said methane product stream.
 20. The process of claim 19,wherein the nitrogen selective adsorbent of step (a) is a CTS-1 zeolitecharacterized as having a pore size of approximately 3-4 Angstrom unitsand a composition in terms of mol ratios of oxide as follows: 1.0±0.25M_(2/n)O:TiO₂:ySiO₂:zH₂O wherein M is at least one cation having avalence n, y is from 1 to 100, and z is from 0 to 100, said zeolitebeing characterized by the following X-ray diffraction pattern.D-spacings (Angstroms) I/I₀ 11.4 ± 0.25 Very Strong 6.6 ± 0.2Medium-Strong  4.3 ± 0.15 Medium-Strong 3.3 ± .07 Medium-Strong 2.85 ±.07  Medium-Strong